The AK-47 has always enjoyed a reputation of rugged reliability. It is long-stroke gas operated, using the gas from the barrel to push a piston attached to the bolt carrier, thus operating the action. The gas tube is fairly large and is visible above the barrel with ports or vents to allow the excess "dirty" gas to escape without affecting the action. The AK-47 is often built with generous clearances, allowing it to function easily in a dirty environment with little or no maintenance. This makes it reliable but less accurate. It is very simple to disassemble and clean, and easy to maintain.
The M16 uses a direct impingement (DI) gas system, similar to normal gas operation in principle, but unique in operation. The gas is sent from the barrel, through the gas tube, directly to the inside of the receiver so it can push on the bolt carrier itself. This means that the gas alone impinges upon the bolt carrier. This design is much lighter and more compact than a gas-piston design. However, this design requires that combustion byproducts from the discharged cartridge be blown into the receiver as well. This quickly accumulating carbon and vaporized metal build-up within the receiver and bolt-carrier negatively affects reliability and necessitates more intensive maintenance on the part of the individual soldier. The DI operation increases the amount of heat that is deposited in the receiver while firing the M16 and causes essential lubricant to be "burned off". This requires frequent and generous applications of appropriate lubricant. Lack of proper lubrication is the most common source of weapon stoppages or jams.
The original M16 fared poorly in the jungles of Vietnam and was infamous for reliability problems in the harsh environment. As a result, it became the target of a Congressional investigation. The investigation found that:
- The M16 was billed as self-cleaning (when no weapon is or ever has been).
- The M16 was issued to troops without cleaning kits or instruction on how to clean the rifle.
- The M16 and 5.56x45mm cartridge was tested and approved with the use of a DuPont IMR8208M stick powder, that was switched to Olin Mathieson WC846 ball powder which produced much more fouling, that quickly jammed the action of the M16 (unless the gun was cleaned well and often).
- The M16 lacked a forward assist (rendering the rifle inoperable when it jammed).
- The M16 lacked a chromed barrel and chamber, causing corrosion problem, contributing to case swelling and extraction failures. (This was considered the most severe problem and required extreme measures to clear, such as inserting the cleaning-rod down the barrel and knocking the spent cartridge out.)
When these issues were addressed and corrected by the M16A1, the reliability problems decreased greatly. According to a February 1968 Department of Defense report, the M16A1 rifle achieved widespread acceptance by U.S. troops in Vietnam. Only 38 of 2100 individuals queried wanted to replace the M16A1 with another weapon. Of those 38, 35 wanted the CAR-15 (a shorter version of the M16) instead. In March 1970, the "President’s Blue Ribbon Defense Panel" concluded that the issuance of the M16 saved the lives of 20,000 U.S. servicemen during the Vietnam War, who would have otherwise died had the M14 remained in service. However the M16 rifle's reputation continues to suffer.
After the introduction of the M4 Carbine, it was found that the shorter barrel length of 14.5 inches also has a negative effect on reliability, as the gas port is located closer to the chamber than the gas port of the standard length M16 rifle: 7.5 inches instead of the 13 inches. This affects the M4’s timing and increases the amount of stress and heat on the critical components, thereby reducing reliability. In a 2002 assessment the USMC found that the M4 malfunctioned three times more often than the M16A4. Thereafter, the Army and Colt worked to make modifications to the M4 in order to address the problems found. In tests conducted in 2005 and 2006 the Army found that on average, the new M4s and M16s fired approximately 5,000 rounds between stoppages.
The newest version of the M16 in U.S. service is the HK416 which uses a proprietary gas system derived from the HK G36, replacing the direct impingement gas system used by the standard M16/M4. The HK system uses a short-stroke gas piston driving an operating rod to force the bolt carrier to the rear. This design prevents combustion gases from entering the weapon's interior, a shortcoming with direct impingement systems. The reduction in heat and fouling of the bolt carrier group increases the reliability of the weapon and extends the interval between stoppages. The short-stroke gas piston require less maintenance and cleaning. It reduces operator cleaning time and stress on critical components. "Improving the service interval requirements provides a major benefit to soldiers that may not have the ability or opportunity to thoroughly clean their rifle. Also, the design of the external gas piston system is less susceptible to build up of other contaminants in extreme environments."
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